Even though it has been more than twenty-five years since the discovery of HIV, an effective preventative vaccine remains elusive. Current candidate vaccines to HIV-1 fail to provide protection and in many cases actually enhance infection. This has been attributed to the inherent difficulties of confronting a virus infecting the cell that is the key component of immune system and the challenges of a pathogen with great diversity and rapid mutation. More critically, these vaccines were developed based on conventional views of virus infection that did not reflect a sufficient understanding of the correlates of protection against HIV-1. Improving such understanding is essential to any successful vaccine development.
Heterogeneity in susceptibility to HIV-1 infection has been observed in several cohort studies. Despite repeated exposures, some individuals do not appear to become infected with HIV-1. Understanding why these individuals can escape HIV-1 infection and how their immune system works will help to reveal parameters of protective immunity and thus the development of effective vaccines and control strategies.
A subset of women in the Pumwani Sexworker cohort, established in 1985 in Nairobi, Kenya, remains HIV-1 seronegative and PCR-negative despite repeated exposure to the virus through active sexwork. Studies showed that this resistance to HIV-1 infection is associated with several alleles of Human Leukocyte Antigens (HLAs) and specific CD8+ and CD4+ T-cell responses to HIV-1 (Alimonti et al., 2996, Immunol Cell Biol 84: 482-485; Alimonti et al., 2005, J Infect Dis 191: 20-24; Hardie et al., 2008, Aids 22: 2038-2042; Hardie et al., 2008, Aids 22: 807-816; Lacap et al., 2008, Aids 22: 1029-1038; Rowland-Jones et al., 1995, Nat Med 1: 59-64; Rowland-Jones et al., 1998, J Clin Invest 102: 1758-1765). HLAs are a group of host proteins that are central in regulating the immune response through the binding and presenting of peptides known as epitopes derived from self and foreign proteins to T cells. The genes coding for HLAs are extremely polymorphic, resulting in a diversity of HLA alleles with variable ability and affinity for the self and pathogenic proteins in the population. This genetic diversity ensures that no pathogens can escape detection at the population level. The contribution of different HLA alleles to virus control varies because of differences in antigenic recognition. The association of HLA alleles with different outcomes of HIV-1 infection are most likely due to the differences in the antigenic peptides or epitopes of HIV being presented and the resulting immune responses that are engaged following immune recognition. Therefore, differences in the recognition of peptides/epitopes between HLA alleles associated with different outcomes of HIV-1 infection might point to a vital clue for developing an HIV-1 vaccine. The iTopian™ antigen discovery system, a novel biochemical CTL epitope discovery system, uses an MHC-peptide complex-specific antibody to assess MHC-peptide binding, relative affinity and complex stability. It permits rapid screening of large peptide libraries for multiple HLA Class I molecules (Luo et al., 2011, J Virol). In preliminary work using the iTopia epitope discovery system combined with IFN-γ CD8 ELISPOT™ assays, 616 9-mer peptides overlapping Gag of HIV-1 subtype A and D for two HLA alleles associated with different outcome of HIV-1 infection were screened. A*01:01 is significantly associated with HIV-1 resistant women (p=0.016, odds ratio: 1.7, 95% Cl: 1.1-2.7) and slower rate of seroconversion (FIG. 1-A), while B*07:02 is associated with susceptibility to HIV-1 infection (p=0.035, odds ratio: 0.38, 95% Cl: 0.14-1.1), rapid seroconversion (FIG. 1-B) in the Pumwani Sexworker Cohort, as well as high viral loads and rapid disease progression in several different populations. As expected, the gag epitopes of A*01:01 do not overlap with the epitopes of B*07:02. However, to our surprise, B*07:02, a allele associated with rapid seroconversion and disease progression, binds 29 peptides spanning the entire gag peptide with high to moderate affinity and low off-rate, whereas A*01:01 only binds to one peptide with relatively high affinity and normal off-rate, and with weak binding to 2 other peptides. Contrary to the conventional view of protective immunity that the tried (and failed) HIV-1 vaccines followed, which is a pan and strong immune response to several HIV-1 proteins (Nature (2007) 499: 390; AIDS Alert (2003) 18: 43-45; McCarthy 2003, Lancet 361: 755-756; Pal et al., 2002, J Virol 76: 292-302; Plotkin, Hum Vaccin 6; Vaccari et al., Expert Rev Vaccines 9: 997-1005; Wilyard, Nature 466: S8), the allele, which recognizes more epitopes and generates strong IFN-gamma ELISPOT responses, is associated with a bad outcome to HIV-1 infection.
At least two things can be learned from this observation: a) since the pan, strong immune responses do not provide protection, an anti-HIV-1 vaccine must not induce them; b) an anti-HIV vaccine must be selective and not target entire HIV-1 proteins. What should be the target? The A*01:01 gag epitope provided a clue. The only gag peptide recognized by A*01:01 with relative high affinity and normal off-rate (ED50:1.211E-5, half life: 0.995 h) is a 9-mer peptide covers the protease cleavage site at p17/p24 (Luo et al., J Virol 86). This region is relatively conserved among major HIV subtypes (A1, B, D, G). We tested 8 peptide variants of these subtype consensus and found that A*01:01 can bind to all of them with similar affinity and off-rates (ED50: 4.3E-6 to 1.21E-5, half life: 0.385 to 1.298 h). Why is this region important for HIV-1? The protease of HIV-1 is a small 99-amino acid aspartic enzyme that mediates the cleavage of Gag, Gag-Pol and Nef precursor polyproteins. The process is highly specific, temporally regulated and essential for the production of infectious viral particles. A total of twelve proteolytic reactions are required to generate a viable virion.